This invention relates to a hydraulic system employed in working vehicles performing for example goods loading, mowing, reaping, earth moving operations and the like.
More particularly the present invention relates to a hydraulic system for use in a working vehicle which comprises a hydraulic power steering mechanism, including a power cylinder, and a hydraulic lift mechanism including a lift cylinder, the power cylinder, and lift cylinder being powered by a single pump driven by engine.
In general, such hydraulic system is fashioned such that it comprises a flow divider means for dividing output flow of a single pump into two flows one of which is directed towards the power cylinder and the other of which is directed towards the lift cylinder. Because it is required for a stable and safe steering of vehicle to turn vehicle steering wheels or front wheels with a predetermined constant rate, such flow divider means is fashioned in many cases so that it operates to secure a predetermined constant flow or flow rate for the power cylinder so as to operate such cylinder with a predetermined constant speed or rate. Thus, flow control valve means is provided which divides output flow of a single pump into a predetermined constant flow to be directed towards power cylinder and a surplus flow in excess of such constant flow. Lift cylinder of a hydraulic lift mechanism is thus operated by the surplus flow having a value substracted the predetermined constant flow from pump output flow.
Flow control valve means of the type set forth above does not provide any flow in its surplus flow outlet port when output flow of a single pump is less than a predetermined constant flow to be secured at the constant flow outlet port of such valve means. In a hydraulic system in which such flow control valve means is employed, lift cylinder of a hydraulic mechanism cannot thus be operated when engine speed is very low so that output of a single pump driven by engine is less than a predetermined constant flow to be secured for operating power steering cylinder with a predetermined constant rate. Within some low range of engine speed, flow of fluid directed towards the lift cylinder is very small so that such cylinder is operated to extend at a very small rate. Such slow extension of lift cylinder will result in a very slow lifting of working implement to be lifted by hydraulic lift mechanism.
Lifting of a working implement which is connected pivotally to a working vehicle so as to be lifted and lowered by hydraulic lift mechanism is often required even at a low speed of engine and even in an idling condition of engine. For example, such working implement is usually lifted in an idling condition of engine before working vehicle is put out of garage. During a working operation which is performed at a creep speed of vehicle, a quick lifting of working implement is required or at least desirable in response to an emergency. It is thus seen that inconvenience has been experienced.